Chapter 10

1. Chapter 10

2. 10.1 The Chromosome Theory of Heredity • Chromosomes are located in the nucleus • Factors (genes) are found on chromosomes • Sutton discovered that genes are on chromosomes in 1902

3. Chromosome Theory of Heredity • States that genes are located on chromosomes and each gene occupies a specific place on a chromosome • Only one allele is on a chromosome

6. Independent Assortment

7. Gene Linkage • Genes on a chromosome are linked together • Inherited together – THEREFORE they do not undergo independent assortment

8. Linked Genes- genes on the same chromosome – inherited as a package Height Gene A Flower color gene B Flower position gene C

9. Thomas Hunt Morgan • Studied fruit flies – Drosophilia melanogaster

11. Fruit Flies are excellent for genetic studies because: • Reproduce quickly • Easy to raise • Many mutations • Have 8 chromosomes (n=4)

13. Fruit Fly Mutations

14. Thomas Hunt Morgan began to carry out experiments with

15. Gray bodies – G Normal Wings - W Black bodies – g Small wings – w Morgan looked at TWO traits

16. The flies mated….

17. The female laid eggs

18. ggww GGWW P1 x GgWw F1 100%

19. Morgan then mated the F1 back to the recessive parent GgWw x ggww Expected ratio – 1:1:1:1 25% GgWw 25% Ggww 25% ggWw 25% ggww

20. Morgan’s Actual Results 41.5% gray normal 41.5% black small 8.5 % black normal 8.5% gray small

21. Conclusion • Gene for body size and wing color were somehow connected or linked • Can’t undergo independent assortment

22. Linkage Groups • Package of genes that are always inherited together • Chromosome • One linkage group for each homologous pair • Fruit flies – 4 linkage groups • Humans – 23 linkage groups • Corn – 10 linkage groups

23. So linkage groups explain the high percentages (41.5%) but What about the 8.5%??????

24. 17% had new combinations The combinations that were expected would be: Gray normal – GW or Black small - gw

25. P1 G G g g W W w w Dad Mom

26. F1 G g W w

27. g G g g W w w w F1 F1 X F1 Recessive Fruit Fly Heterozygous

28. The Offspring of the Cross g G g g and W w w w F1 F1 41.5 % 41.5 %

29. Genes of the Heterozygous Parent G G g g W W w w The homologous pair copied

30. The homolgous pairs pair up in Prophase and form a tetrad G G g g W W w w

31. When they are lined up they can become twisted and switch genes Crossing Over

32. So you could then have ….. G G g g W w W w switch

33. The other offspring of the cross g G g g and w w W w F1 F1 8.5 % 8.5 %

34. The 17% that had new combinations are known as • Recombinants – individuals with new combinations of genes • Crossing Over – gives rise to new combinations – Prophase I

35. Gene Mapping • Sturtevant – associate of Morgan • Crossing over occurs at random • The distance between two genes determines how often they cross over • Genes that are close do not crossover often • Genes that are far apart – cross over often

36. So…… • If you know the frequency with which crossing over occurs then you can use that to map the position of the genes on the chromosome

38. Frequency of crossover exchange... is GREATER the FARTHER apart2 genes are is proportional to relative distance between 2 linked genesRelative distance is established as...    1% crossover frequency =1 map unit of map distance    1%   CrossOver  Freq   =    1   centiMorgan

39. Sex Linkage • Stevens – made observations of meal worm chromosomes

40. Sex Chromosomes • One pair • Female – XX • Male – XY

41. Autosomes • All the chromosomes except the sex chromosomes

42. Sex Determination

44. Genes on Sex Chromosomes • Sex chromosomes determine a person’s sex • Sex chromosomes also contain genes

45. Sex Linked • A gene located on a sex chromosome • Usually X • Example – Fruit Fly Eye Color • So the gene for eye color is on the X chromosome and not the Y

47. Fruit Fly Sex Chromosomes X X X Y

48. Males Females XRY XrY XRXR XRXr XrXr Red Eyed White Eyed

50. Mutations